Elektorun Kasım Aralık 2020 sayısındaki 2Mhz LCR metrenin Ölçme Prensibindeki Açıklama:
Measurement principle
The impedance (Z) is an important parameter to characterize passive electronic components (resistors, capacitors, inductors). It is a complex number which can be represented by a real part (R) and an imaginary part (X) such that Z = R + jX , or in polar form by the modulus of its impedance and the phase shift between voltage and current:,
Z ∠θ .
To determine impedance it is therefore necessary to measure at least two values (in magnitude and in phase), generally the voltage at the terminals of the component and the current flowing through it.
The LCR meter AU2019 uses, like its predecessor, the self-balanced bridge method with the use of a simple operational amplifier for the current/voltage converter (I-V converter, see Figure 1).
This simple method provides good measurement accuracy at a reasonable cost. Its main disadvantage is a frequency range limited in the high frequencies by the performance of the operational amplifier used.
To obtain a wide impedance measurement range (from a few tens of mΩ to more than 100 MΩ) it is necessary to switch the precision resistor (R) used in the I-V converter. Unfortunately, common analog integrated switches (such as 74HC4052) introduce parasitic elements (mainly capacitance) which will limit the performance at high frequencies.
This is why most similar instruments have their frequency range limited to 100 (or 200) kHz.
It is however possible, by an original design and the choice of high-performance components, to push the high frequency to 2 MHz without excessively higher costs, while maintaining the simplicity of the design.
The chosen solution, shown in Figure 2, is not to switch the four measuring resistors (and thus reducing parasitic capacitances to a minimum) but to have four Amplifier + Resistor pairs, each pair selected to match with the impedance to be measured.
The operational amplifiers used (AD8099 from Analog Devices) have a cut-off frequency of approximately 200 MHz at an output voltage of 2 Vp-p and have a control input to inhibit (mute) the output, exactly what we need for this application. The switches are PhotoMOS solid-state relays manufactured by Panasonic
with a very low product (ON resistance x output capacitance).
Another important point is the choice of the method of generating the test frequency. Nowadays it is easy and inexpensive to use Direct Digital Synthesis (DDS) components for this task, with the advantage that any frequency in the 50 Hz / 2 MHz range can be generated. It is, moreover, easy to generate a signal of the same frequency for the synchronous detector, but with variable relative phase, thanks to a second DDS circuit that is synchronized to the first.
